Update webhook docs

[roznawsk]

Summary

Updates the backend docs to reflect webhook notification batching (js-server-sdk#273, python-server-sdk#83).

What is batching? By default Fishjam sends one webhook POST per event. With batching enabled, it coalesces several notifications produced close together into a single NotificationBatch ServerMessage per POST — fewer HTTP requests and faster delivery, which keeps your backend responsive under load. Receivers just decode the body and iterate; the decoder unwraps the batch, so batched and single notifications are handled the same way.

• Document and recommend batchWebhookNotifications / batch_webhook_notifications when creating a room.
• Switch webhook receivers to the batch-aware decoders decodeServerNotifications / decode_server_notifications, which transparently unwrap a NotificationBatch — a single message and a batch are handled identically.
• production-deployment: replace the fictional signature-verification flow with the real application/x-protobuf decode loop, add Python tabs throughout, and split the auth example into authentication and create-room-and-add-peer (the room is now created lazily before the peer).
• backend-quick-start: drop the misleading "listen for events" bullet.
• Bump js-server-sdk and python-server-sdk submodules.

Verified: yarn build (twoslash type-checking) and spellcheck both pass.

[linear]

FCE-3423

[Copilot]

Pull request overview

This PR updates the backend webhook documentation to recommend/illustrate webhook notification batching and to standardize webhook handling around SDK-provided decoders that return lists of notifications (batched or not).

Changes:

• Document batchWebhookNotifications / batch_webhook_notifications and update room-creation examples to enable batching.
• Add/refresh webhook receiver examples to decode application/x-protobuf webhook bodies into iterable notification lists.
• Refresh production deployment guidance with clearer webhook/auth examples (now split by language tabs).

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
docs/tutorials/backend-quick-start.mdx	Removes an outdated “listen for events” learning bullet to better match the tutorial scope.
docs/how-to/backend/server-setup.mdx	Adds batching recommendation + decoding examples for Node/Python webhooks.
docs/how-to/backend/production-deployment.mdx	Updates production guidance with language-tabbed auth + webhook handling and batching recommendation.
docs/how-to/backend/fastify-example.mdx	Updates Fastify webhook parsing to use decodeServerNotifications and handle notification lists/batches.
docs/how-to/backend/fastapi-example.mdx	Updates FastAPI webhook parsing to use decode_server_notifications and iterate notification lists/batches.
docs/api/reference.md	Adds reference-level guidance on batching and decoder behavior.

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